Depends what you intend to use them for, or what aspects in the game you would want made easier.
IMO, in a primarily PvP focused bracket, everyone should have some sort of customizable targeting addon for the opposing team (PvP Grid/BattleGround Targets for Bgs and Gladius (the other similar one is eluding me) for arena).
I forget the name, but the one that tracks time to flag respawn/base captures in AB is fairly useful.
The curse interface has a list of add ins you can sort by popularity and read their descriptions. This is likely your best start.